Replaceable ink supplies are important components of ink-jet type printers. The ink supply provides ink to a printhead that is carried in what may be called the pen of the printer. The printhead typically includes a plurality of orifices, each orifice having an associated chamber. Ink is channeled to the chamber from the ink supply. During operation of the printhead, ink droplets are fired from the chambers, through the orifices, to a printing medium such as paper.
In thermal-type printheads, the ink droplets are fired as a result of rapidly heating the ink in the chamber by an amount sufficient to vaporize a portion of that ink. The resultant, rapid expansion of the vapor bubble in the chamber forces out of the chamber a correspondingly sized droplet.
Some printer designs separate the ink supply from the printhead, which printhead is normally mounted to a carriage to reciprocate along the width of paper that is advanced through the printer. The supply resides in the printer, and an elongated tube or other means is used for interconnecting the supply to the printhead.
Although the printhead is a reliable and efficient means for firing ink droplets, it carries no mechanism for preventing the free flow of ink through the orifices when the printhead is not operating. As a result, ink supplied to the printhead is usually provided under a slight under pressure or back pressure. The back pressure is large enough to prevent the free flow of ink from the pen, but not so large as to prevent an activated printhead from expelling ink. This range of back pressures will be referred to as the printhead's operating range. As used here, a positive back pressure refers to a pressure within the printhead (or ink supply) that is less than ambient pressure. Thus, an increase in back pressure means an increase in the difference between ambient pressure and the relatively lower back pressure.
The back pressure at the printhead must be maintained within a fairly narrow operating range (to prevent leakage without causing the printhead to fail) despite severe changes in the ambient temperature and pressure that may occur, for example, when a printer is subject to altitude changes during shipping, etc. A large ambient pressure drop, for example, could overcome the back pressure in the printhead and cause ink to leak or "drool" from it. Thus, printers are provided with mechanisms that compensate for such changes. One class of mechanisms, which may be referred to as accumulators, are designed to expand and contract or otherwise compensate for pressure or volume changes inside the pen that are attributable to ambient pressure changes.
Accumulator mechanisms are sometimes supplemented with "bubble generators." A bubble generator is an orifice or tubular member formed in the ink supply reservoir to allow, under certain conditions, fluid communication between the interior of the reservoir and the ambient atmosphere. The opening of the bubble generator is sized to have capillarity or capillary pressure sufficient to retain a quantity of ink in the opening as a liquid seal. The geometry of that opening is such that when the back pressure approaches the limit of the operating range of the printhead, the back pressure overcomes the capillary pressure of the bubble generator and the liquid seal is broken. Ambient air then "bubbles" into the reservoir to reduce the back pressure to an acceptable level. Ideally, when the back pressure is so reduced, ink from the reservoir reenters the orifice to reestablish the liquid seal.
Open-cell foam has been used as a storage medium in ink supplies. The capillary pressure of the foam provides a simple mechanism for providing back pressure for the supply. In this regard, capillary pressure is the pressure applied by a capillary member, such as the connected cells in reticulated foam, to a liquid that it contacts, such as ink. For example, foam material having a capillary pressure of 7 centimeters water column would store the ink in its cells until a suction greater than that pressure is applied to it.
The volumetric efficiency of an ink supply generally means the amount of ink deliverable from the supply reservoir divided by the reservoir volume. When the entire ink supply is stored in foam the volumetric efficiency of the supply suffers because of the presence of the foam material throughout the supply. The solid parts of the foam material fills volume that may otherwise be used to store ink. High volumetric efficiency is desirable for enabling as much ink as possible to be delivered to the printhead (hence to the paper) for a given size ink supply.
Another disadvantage with all-foam type supplies is that the level of available ink in the supply may not be as readily detectable as would be the case if the supply consisted of free ink having a discernable level.
Ink supplies that are contained free (that is, without the use of porous, absorbent material, such as the foam mentioned above) offer high volumetric efficiency along with a free-ink surface for detecting the ink level. Mechanisms necessarily associated with such supplies for regulating back pressure, however, tend to be complex and relatively difficult to manufacture.
The present invention is directed to an ink supply contained in a manner that combines foam and free ink storage to provide high volumetric efficiency, back pressure regulation to protect against ink leakage, and a generally lower cost, easy-to-manufacture assembly.
In a preferred embodiment of the invention, the container comprises a reservoir that is divided into two parts. One part stores free ink, and another part holds porous, absorbent "accumulator" material that stores ink and has a capillary pressure sufficient to provide back pressure in the supply.
Porous wicking material is in the reservoir, arranged to be in fluid communication with the free ink and with the ink in the accumulator material. The wicking material delivers both the free ink and ink in the accumulator material to an outlet in the reservoir.
A bubble generator is provided to connect the free-ink part of the reservoir to the ambient atmosphere. The bubble generator is designed to have a capillary pressure that is significantly higher than the capillary pressure of the accumulator material. As such, ink removed from the supply is first drawn from the accumulator material, which thus allows that drained material to thereafter act as an accumulator in the event of severe pressure or temperature changes as mentioned above.
After ink is removed from the accumulator material, ink is drawn via the wicking material from the free-ink supply. The bubble generator permits ingress of air as this ink is removed, thereby ensuring that the back pressure in the supply does not rise so high as to cause the printhead to fail.
The volumetric efficiency of the present supply is enhanced in a number of ways. For example, the significantly greater capillary pressure of the bubble generator as compared to that of the accumulator material ensures that nearly all of the ink in the accumulator material will be removed before the free-ink supply is depleted. Thus, the porous material of the accumulator will hold very little "stranded" ink when the supply is otherwise fully depleted.
In one embodiment of the invention, the porous material used for the accumulator material is selected to be very wettable (i.e., a zero or near-zero contact angle between the ink and surface of the material). This facilitates movement of ink from the accumulator to further minimize the amount of stranded ink.
The porous wicking material is selected to have a capillary pressure that is higher than that of both the accumulator material and the bubble generator. As such, the wicking material remains saturated with ink at least until all of the available accumulator ink and free ink is removed from the supply. Consequently, the wicking material provides a reliable mechanism for ensuring delivery of ink out of the supply.
The supply of the present invention is adaptable to be remote from the printhead, and configured with an outlet that receives a fluid interconnect mechanism for conducting ink from the supply to the printhead. A negative (suction) pressure is applied via the interconnect to remove the ink. In one embodiment, the capillary pressure of the wicking material is selected to be great enough so that ink is retained within the wicking material after removal of the accumulator ink and free ink. This design ensures that the wicking material will remain at least partly saturated, which is necessary in some instances to ensure fluidic coupling with the interconnect. This design may be useful, for example, when the fluid interconnect is periodically made and broken throughout the useable life of ink supply.
In another embodiment, the capillary pressure of the wicking material, while greater than that of both the bubble generator and the accumulator material, is established to be low enough to permit the suction of the interconnect to drain ink from the wicking material. Such a design may be useful, for example, in instances where the fluid interconnect is made to a full supply and not broken until the entire supply is depleted. It will be appreciated that this approach enhances the volumetric efficiency of the supply by increasing the amount of deliverable ink (that is, to include what is stored in the wicking material) for a given size of supply container.
As another aspect of this invention, there is provided a method for optimizing the design of the accumulator part of the ink supply so that the accumulator goals (providing back pressure regulation to avoid ink drool despite extreme changes in ambient pressure) are met with the smallest amount of accumulator material required. Minimizing the amount of accumulator material thereby minimizes the amount of ink that may be stranded in the accumulator material, which in turn increases volumetric efficiency.
Inasmuch as a part of the present supply contains free ink, the level of ink remaining in the supply is available for detection.